[libcxx] [test] Split the TEST_HAS_SANITIZERS define into TEST_IS_EXECUTED_IN_A_SLOW_ENVIRONMENT

This makes its role clearer. It's plausible that one may want to manually
define TEST_IS_EXECUTED_IN_A_SLOW_ENVIRONMENT when running the tests in
some environments - in particular, it seems to be necessary to use the
higher tolerance timeouts if running the tests on Windows runners
on Github Actions.

Also add the descriptive comment in one file where it was missing.

Differential Revision: https://reviews.llvm.org/D131484